Most flying or gliding animals rely on a symmetrical pair of wing-like
structures to generate lift, the force that opposes gravity and keeps the
flier aloft. But `flying' tree snakes have no wings or any other obvious
structures that might help them fly, explains Jake Socha, a biologist at
Argonne National Laboratory, Illinois. Little was known about these enigmatic
creatures until Socha, then a graduate student at the University of Chicago,
set out for Singapore in search of the elusive paradise tree snake
(p. 1817 and
p. 1835). Aside from
a desire to see up close the marvel of a flying snake in action, he was
compelled by the question: How do they do it?

To answer this, Socha needed to describe the kinematics of snake flight.
Luckily, with the help of local volunteers, catching paradise tree snakes
wasn't too hard. But capturing their flights on videotape `was a nontrivial
process,' says Socha. He coaxed the stubborn snakes to jump from 10 m high
towers set up in the Singapore Zoological Gardens. Tony O'Dempsey, an expert
on photogrammetric techniques, helped Socha extract 3-D flight information
from the synchronized recordings of two video cameras. `Finding Tony was a
stroke of luck', says Socha; the 3-D information was crucial in reconstructing
the flying snakes' aerial trajectories, speed and body postures.

Socha found that paradise tree snakes are true gliders, and pretty good
ones at that. Rather than simply parachuting to the ground, these
cylindrical-bodied snakes somehow generate enough lift to carry them across a
substantial horizontal distance. Socha suspected that the unusually dynamic
flight behavior of the snakes might be responsible; while flying, the snakes
create distinctive S-shaped aerial waves that travel from head to tail. To
test this, he correlated the snakes' wave amplitude and frequency with flight
performance variables. He found that snakes with the greatest combination of
body length and wave amplitude (relative to body size) travelled fastest; but
the largest snakes were not necessarily the ones making the biggest waves.
While wave amplitude is important, Socha found that the frequency of these
waves appears to play little or no role in producing aerodynamic forces during
flight. So why do snakes undulate so frequently? Socha suggests that it may
provide stability, keeping the snakes from spinning out of control.

Given that body size influences flight performance in other animals, Socha
correlated snake body size with flight parameters and found that smaller
snakes are generally better gliders. He also found that the paradise tree
snake is a better glider than its larger, stockier cousin, the golden tree
snake. But Socha noticed that paradise tree snakes tend to flatten their body
more during flight, suggesting that body shape may also be a key factor in
snake flight aerodynamics.

Socha has only begun to unravel the mysteries of snake flight. He is now
working on physical and computational models to test more specifically how a
snake's shape, size, and behavior influence flight dynamics. Ultimately, he
hopes to understand how differences in morphology and flight performance might
influence ecological differences between flying snake species. This will
require more field studies, but luckily for Socha, `the thrill of watching
snakes fly is something that never gets old.'